Catalyst composites containing zeolites are well known and are commonly used to convert hydrocarbons. The use of zeolite L in combination with other catalytic components is known to be an effective catalyst for reforming hydrocarbons. Reforming converts C6 and C7 light paraffinic hydrocarbons into C6-plus aromatic hydrocarbons, such as benzene and toluene. The C6-plus aromatic hydrocarbons are valuable as high-octane gasoline blending components and as intermediates in the production of commodity petrochemicals.
In reforming, a paraffinic hydrocarbon feedstock contacts a zeolite L-containing catalyst in the presence of hydrogen at an elevated temperature. Paraffins in the feedstock react to form the desired aromatic hydrocarbon product, of course. But other reactions form coke, an undesired carbonaceous byproduct that accumulates in deposits on the catalyst and deactivates the catalyst. Well known steps to regenerate deactivated zeolite L-containing catalyst remove these coke deposits by contacting the coked catalyst with a gas containing molecular oxygen at an elevated temperature (typically above 450° C. (842° F.)) to burn the coke. However, it is also well known that such coke burning significantly worsens the activity, conversion, and selectivity of the zeolite L-containing catalyst, because it agglomerates one of the other components on the catalyst, which is typically an IUPAC Group 8–10 (VIII A) metal. As a result, additional subsequent steps must be added to the regeneration process to redisperse the catalytic metal. These redispersion steps are well known and usually involve contacting the catalyst with a halogen-containing gas, often in the presence of molecular oxygen and water vapor.
Eliminating the redispersion step is desirable for several reasons. First, the presence during redispersion of molecular halogen, such as chlorine, and/or its compounds, such as hydrogen chloride, with water can corrode or otherwise damage equipment used in regeneration. Second, the halogen-containing materials present during redispersion are volatile and the environmental risk arising from their accidental release to the atmosphere is more and more undesirable. Third, prolonged exposure of the catalyst to the elevated temperatures used in redispersion can damage certain select physical properties, such as the surface area, of the catalyst. Fourth, redispersion is time-consuming and inefficient, since it would be a better use of the capital investment in the catalyst if the catalyst were being used to reform hydrocarbons rather than undergoing redispersion, provided that the catalytic metal remains dispersed.
Using ozone at low temperature to regenerate zeolite catalysts has had unpredictable results. Failure or only partial success occurred when the catalyst lacked a metal, as described in U.S. Pat. No. 5,183,789 at col. 2, lines 37–57. Since metals catalyze the conversion of ozone to oxygen, the prior art expected that adding a metal to a catalyst would worsen the chances for a successful regeneration. That in fact happened with a paraffin conversion catalyst as described in the article by C. R. Vera, et al., in Catalyst Deactivation 1999, Studies in Surface Science and Catalysis, vol. 126, at pages 381–388. But in contrast in U.S. Pat. No. 5,183,789, a reforming catalyst containing a metal was successfully regenerated; see col. 2, line 57 to col. 3, line 41, and col. 4, lines 32–38.
A method for regeneration of a zeolite L containing catalyst is sought which eliminates the need for redispersion.